Energy-dispersive X-ray spectroscopy (EDS) is X-ray spectroscopy by means of energy detection, as opposed to the different method of X-ray spectroscopy by means of wavelength detection (WDS). EDS is commonly used in electron microscopes and in X-ray fluorescence (XRF) spectrometers. In the latter case, the specific term for the method is Energy Dispersive X-Ray Fluorescence (EDXRF), i.e. XRF by means of EDS. In the following description, the EDXRF utilization is used as example, but it is to be understood that the present invention relates to the detector of the system, and this might be used generically in all EDS. The invention can also be used for other high-energy photon spectroscopy, and also even for X-ray or γ-ray imaging.
A major limiting factor in the performance of an EDS system is the detector, and two important parameters that determine the quality of a detector are the energy-resolution and the detection throughput. Detection throughput is defined herein as a parameter combining the active area coverage of the detector with the speed at which the X-rays are processed, i.e. high throughput occurs when the active area coverage is large in combination with the ability to also process the X-rays at high speed across this entire area.
Almost all EDS sensors are based on some form of a semiconductor element (diode) as the sensor, i.e. the material that converts the X-ray to an electric signal. They all operate by the same basic principle: an incoming X-ray that reacts with an atom in the sensor gives rise to an ionization process that creates an electric charge which, in the presence of an electric field, will drift to the anode of the sensor where it can be picked up by an electronic amplifier. The amount of charge produced in such event is proportional to the energy of the incoming photon.
The sensor is a crucial element in determining the quality of the X-ray detection. One of the main challenges in obtaining high energy resolution is that the sensor also needs to cover a certain large physical area in order for it to be efficient i.e. have high throughput. For some conventional sensor types in recent years, however, the energy resolution usually degrades in proportion to the active sensor area, owing to an increase in its capacitance and dark-current. For this reason, such sensors may have been cooled with cumbersome cooling systems, in order to minimize the dark-current. Today's most popular sensor type, the Silicon Drift Detector (SDD), however, solves the problem of capacitance through an asymmetry between the sensor's cathode and anode. The cathode is kept large which makes the area coverage large as well, whereas the anode is made small which creates the low capacitance. With low capacitance, the dark-current is less of a problem because its effect can be minimized by using a high bandwidth in the subsequent amplification filter. The latter comes with yet one more advantage, namely allowing for higher speed detection.
But there is one disadvantage also with the SDD, namely that all the X-ray originated signals picked up in the large cathode area need to be “squeezed through” the small area of the anode, such that one single amplifier needs to cope with the entire X-ray flux of the large detection area. So even if the throughput for the SDD is more than satisfactory in comparison with other sensors used today, it still represents a major limitation.
The most crucial parameters determining the quality of an X-ray spectrometer detector are thus:
1. The energy-resolution (limited by capacitance and dark-current), and,
2. The X-ray rate throughput capability.
The SDD is optimal only in respect to the first, whereas the invention presented herein will improve the X-ray throughput capability but not at the expense of the energy-resolution.
Usually, X-ray spectrometer detector developments are directed to obtaining as large detection coverage-area as possible while, at the same time, maintaining energy-resolution to a desired value. The current industry standard for this is typically better than ˜150 eV (full width at half maximum—FWHM), and state-of-the-art SDDs can accomplish this at detection coverage-areas in the range 10-100 square mm, and with a detection-rate capability of ˜1×106 X-rays per second for that coverage area. This limitation in throughput is in part dictated by the bottleneck caused by the fact that the sensor is in effect, just one large pixel, i.e. all photons must pass through the one single anode to get to the first amplifier and onward to the downstream processing stages. This means that if two or more photons strike the sensor near simultaneously, the first amplifier will not be able to separate between the photons, and the two signals will (partly) be piled up on the back of each other and appear to be just one signal with a combined energy. To minimize such pile-up effect, the X-ray rate must be limited so that the likelihood for it to occur becomes very small. It should be noted that use of what is effectively only a single pixel is not a problem where X-ray spectrometry is concerned, since generally all that is important is the charge generated by the photon and not its location. So the fact that two photons strike the sensor at different locations simultaneously does not require their locations to be discriminated, but it does require that the equivalent energy signal corresponding to each photon be discriminated.
In such SDDs, the electric signals resulting from all the X-rays impinging across the entire coverage-area need to pass through just one single electrode, and one single electronic channel, i.e. a front-end amplifier and a subsequent downstream signal processing chain, must handle all the signals, without the possibility for parallelism. The front-end amplifier in all of these kinds of detectors must have a certain bandwidth limitation in order to obtain the required energy-resolution, and it is therefore a limit to how high a signal flux can pass through one single channel.
Therefore, considering the same detection coverage-area, it would be expected that higher rates could be obtained by fragmenting the sensor into a 2D array of smaller pixels, each having their own electrode and their own signal processing channel. For example, considering a 100 mm2 detector coverage-area fragmented into 1 mm2 pixels, and thus having 100 parallel signal processing channels, the potential signal-rate capability would increase by the same 100 times, which is significant. This would in theory be accomplished because smaller detector segments (pixels) will actually allow for smaller capacitance and smaller dark-current per channel, and with sufficiently small pixels it should in theory be possible to achieve the required energy-resolution without even using the silicon-drift principle i.e. even regular cubic diodes ought to suffice. The latter should also simplify the sensor structure.
While this should all be true in theory, in practice no such detector has been realized owing to the difficulties in implementation. It is to the actual realization of such a detector that the invention is directed.
Pixel Detectors
The image capture device in digital photo cameras can be said to be an array of pixel detectors although the denomination CCD or CMOS sensor is rather used. The term pixel detector is more common when detecting other photons/particles than visible light, in particular in high-energy-physics detectors. In photographic capture devices the sensing elements (diodes) are usually combined with some degree of front-end electronics inside of the pixels in the same die. When the term ‘pixel detector’ is used, the sensing pixels (diodes) are usually located separately on one die, whereas the corresponding front-end electronics pixels are located in a separate die. Because of the 2D matrix arrangement of the pixels, the pixel detector requires a special method in order to interconnect the sensor pixels with the front-end electronics pixels, called bump-bonding, as shown pictorially in FIG. 1.
In the following description, the term ‘detector’ refers to the pair of 1) a sensor (typically crystalline silicon), being the material converting the X-rays into electrical signals, and 2) the first-stage electronics which, at the very least, processes the electric signals from its very weak analog form into a suitable much stronger analog form which more conveniently can be transferred to a second-stage electronics. In its simplest form, this first-stage electronics could be just an amplifier, but in the invention there is more functionality. Also, in this invention, the first-stage electronics is represented in a single-die integrated circuit (IC).
In the definition used here, the second-stage electronics is not a part of the detector. This second-stage electronics typically contains data acquisition electronics (DAS) that converts the detector-data into digital form (A/D conversion), before conveying it to a computer for analysis, and handles detector control functions.
In the following description the notion “naked-die IC” can be used for the first-stage electronics; and the notion “DAS” for the second-stage electronics.
Thus, the pixel detector is typically a two-layered sandwich, allowing 2D dense integration of pairs of sensor-element and first-stage electronics. In FIG. 2, the X-rays enter from the top, are to an electric signal (charge) in the upper-layer sensor (confined in just one of the pixels), are transferred via the bump connection to the corresponding pixel in the lower-layer naked-die IC for the first-stage electronics processing. This sandwich corresponds to the sensor and the first stage electronics, as shown schematically in FIG. 2.
Such a configuration is shown, for example, in WO 2009/072056, which discloses a monolithically integrated crystalline direct-conversion semiconductor detector comprising an unstructured semiconductor layer having a polished surface on an anode-faced side. The opposite side is attached to an electrically conductive intermediate layer. The polished surface is contacted at ultra-fine pitch with metal bumps of a bumped. CMOS wafer given by a crystalline semiconductor layer, the metal bumps being bonded to the anode-sided surface of the semiconductor layer.
It is, however, important to distinguish between imaging detectors of the kind described in WO 2009/072056 and the type of detector shown schematically in FIG. 1 with which the present invention is concerned. Thus, a pixel detector used for capturing a photographic image is bombarded with light that is reflected from a target and focused by a lens. The light that strikes the pixels of the pixel detector includes thousands of photons and each pixel, in effect, stores an integrated charge corresponding to the total number of photons that reach the pixel during the small time interval that the camera shutter is open. The energy of any single photon is not significant and therefore the event that gives rise to each photon emission does not need to be detected.
In contrast to such a system, X-ray spectroscopy directs a source of high energy electrons from an X-ray source to a target. If the electron has enough energy it can knock an orbital electron out of the inner electron shell of an atom of the target, and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted. This process produces an emission spectrum of X-rays at a few discrete wavelengths, sometimes referred to as the spectral lines that depend on the element used and thus are characteristic of the element. If the target is a composite of different elements, each element will emit spectral lines of a different characteristic wavelength.
An X-ray spectrometer does not produce an X-ray image of a target but rather analyses the different characteristic wavelengths emitted thereby, in order to identify the composition of the target. It does this by counting the number of X-ray photons emitted thereby for each characteristic wavelength, so that the relative number of X-ray photons for each characteristic wavelength provides a quantitative measure of the composition of the target. Thus, pure copper emits X-ray photons whose energy is approximately 8.05 keV while iron emits X-ray photons whose energy is approximately 6.4 keV. In an X-ray spectrometer the X-ray photons emitted by the target are scattered. Thus, unlike a conventional photographic detector where each pixel is imaged by a unique, identifiable point of the target, the X-ray photons emitted by the target in an X-ray spectrometer are scattered and cannot be mapped to any specific coordinate of the target. But the cumulative number of X-ray photons of each characteristic image provides a measure of the composition of the target. So the relative number of X-rays having energy of 8.05 keV and 6.4 keV, allows determination of the relative proportions of each element.
Thus, the pixels in the detector of an X-ray spectrometer must respond to each discrete X-ray photon that strikes it and record its energy, and the detector must keep a record of the cumulative number of X-ray photons of each characteristic energy. Each X-ray photon striking a pixel gives rise to a charge whose value is proportional to the energy of the photon, and this may easily be measured as current or voltage using suitable pixel electronics.
However, while the quantum physics that determines the characteristic energy of an X-ray photon is determinate and inviolable, being a function of the energy difference between the quantum levels of two electron orbits of the target, the X-ray energy that is actually registered by a pixel is subject to indeterminacy owing to slight differences in the charge accumulated by the pixels of the detector even when nominally they are hit by X-ray photons emitted by the same element. So, for example, the X-ray energy (Kα) of copper is 8.046 keV while that of zinc, which is the adjacent element in the next group of the Periodic Table, is 8.637 keV, a difference of nearly 6%. As atomic number increases, the relative difference in X-ray energy between adjacent elements in the same row of the Periodic Table decreases. Thus, the X-ray energies of tin (Si) and antimony (Sb) are 25.271 and 26.359 keV, respectively, i.e. a difference of only 4.305%. Such a difference may well be smaller than the manufacturing tolerance of the components used in the pixel electronics, which might therefore register an X-ray photon as having been emitted by the wrong element and allocate the photon count to an incorrect bin.
This requires that each pixel in the detector be calibrated so that any slight differences in the relative sensitivities of pixels and their associated electronics may be nullified, thereby producing consistent normalized responses for every pixel in the sensor array.